Optimization of the Pr doping in the (Bi1.7Pb0.3)(Sr2-xPrx)CuO6+ ...

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Nov 15, 2013 (3 years and 8 months ago)

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Optimization of the Pr doping in the (Bi
1.7
Pb
0.3
)(Sr
2
-
x
Pr
x
)CuO
6+


superconducting series


Y. C. Chu
1
,

H.
-
C. I. Kao
1
,

D. C. Ling
2
,

H.

S.

Sheu
3

and

T
.

S.

Chan
3

1
Department of Chemistry, Tamkang University,

Tamsui 251,

Taiwan

2
Department of Physics,
Tamkang U
niversity,

Tamsui 251,

Taiwan

3
National
Synchro
tron Radiation Research Center
, Hsinchu 300, Taiwan

*
virus777787
@
hotmail
.
com



Abstract

A series of Bi
-
2201
having

the nominal composition of (Bi
1.7
Pb
0.3
)(Sr
2
-
x
Pr
x
)CuO
6+δ

with
0
.050



x


0.50 was

prepared by the solid state reaction method.

All of them
have
the
orthorhombic
phase with a space group of
Amaa. Orthorhombicity, 2(a


b)/(a + b),
decreases with
increasing

the amount of Pr content.
The h
ighest T
c

(20.1 K) is

found in the
sample with
x = 0.4
0, which has an optimal hole concentration of 0.278(2) analyzed by an
iodometric titration

method
.

Hole concentrations with respect to the amount of Pr
substitution
.


Keywords:
Bi
-
2201, mixed valence, unit cell parameters,
superconductor
.


1.
Introduction

C
uprate

su
pe
rconductor
s have

interesting

and rich

characteristics of

the normal

state

conduction

behavior
. Basically,
all

s
uperconducting cuprates

have

active and non
-
active layer

intergrowth

in the

crystal

structure

[1].

T
he superconducting phase with a general formula
had been discovered in the Bi
2
Sr
2
Ca
n
-
1
Cu
n
O
6+


system [
2

4
]. They are the Bi
-
2201 (
n =

1),
the Bi
-
2212 (
n
= 2), and the Bi
-
2223 (
n
= 3), and their superconducting transition
temperatures are about 20, 80 and 1
00 K, respectively.

Bi
2
Sr
2
CuO
6+


(
BSCO
)

has a simple
structure with a single CuO
2

layer
.

The doping dependence on T
c

has been studied
extensively

since the hole concentration is easily changed by

partial

substitution

of Sr

by La
.

By
reduce the hole concent
ration
,

optimal T
c

of
Bi
2
(Sr
2
-
x
La
x
)
CuO
6+
δ

(BSLCO)

rolled up to 38
K
[
5

8
]
.

Bi
-
based superconductor posses

incommensurate modulation structure

due to the
extra

oxygen insertion along the unit cell b
-
axis [
9

11
].

which affects the superconductivity

and the

n
ormal
-
state transport properties

of the samples drastically

[
12

1
3
]
.

Pb

doping

leads
to the decrease of the modulation in (Bi
2
-
x
Pb
x
)Sr
2
CuO
6+
δ

(BPSCO) [14].
Co
-
doping of

Pb and
La
,

opti
miz
ation of the hole

concentration

while
reducing the

modulation
,
(Bi
2
-
x
Pb
x
)(Sr
2
-
y
La
y
)CuO
6+δ

(BPSLCO) has a maximum T
c

up to 40.3

K

[
1
5
].


In th
e 123 system
,
a series of 90 K superconductors with formulas as RBa
2
Cu
3
O
7
-
δ

were
observed, where R = rare earth, expect Ce, Pr and Tb [
16
]. All of Ce, Pr and Tb elements
have the 4+

valence state that is probably the reason that they can not be replaced into R site.

2


2.

Experimental

Bulk samples of
(Bi
1.7
Pb
0.3
)(Sr
2
-
x
Pr
x
)CuO
6+


were prepared
by
a

conventional solid
-
state
reaction method.

The
Pr
2
O
3

was preheated at

1000°C for
24

h and k
ept in a desiccator prior

to use. Stoichiometric amount

of Bi
2
O
3
, PbO, SrCO
3
, Pr
2
O
3

and CuO were weighted and
ground thoroughly
. Mixed powder was calcined at 805
°C

for

26

h in a box
furnace

with
five

intermittent grindings.

It was reground and pressed into

pellets,
sintered at
805
°C

for
24

h
and quench
ed

to

room temperature.

S
amples were checked for single phase formation by a
Bruker


MXP3

X
-
ray

D
iffractometer

equipped with a graphite
monochromator
.

Unit cell
parameters were determined by the Rietveld refin
ement method
.

GSAS (General Structure
Analysis System) developed by Larson and Dreele from Los Alamos National Laboratory
was employed for the structure analysis

[17
]
.

T
c

was found from the resistivity

versus
temperature curve measured by a standard

4
-
prob
e method. Oxygen stoichiometry and the
hole

concentration of the compound were determined by an

iodometric titration method [1
8
].

The

relative standard deviation obt
ained from titration

result
s for each sample is less than
1
%
.


3.

R
esult
s

and

di
scussion

Po
lycrystalline samples of
(Bi
1.7
Pb
0.3
)(Sr
2
-
x
Pr
x
)CuO
6+


were prepared with
0
.050



x


0.5
0
.

Fig. 1
shows the X
-
ray diffraction patterns in the 2
θ

range

between
0
°

and 50
°

for all
the compositions in
(Bi
1.7
Pb
0.3
)(Sr
2
-
x
Pr
x
)CuO
6+


series.

All of them are single
-
phase

compounds with orth
orhombic crystal system. Purity

of the samples were carefully examined
by XRD

patterns in the range of
3
0
° ≤
2
θ ≤
3
5
°
. Impurity

phases

Sr
CuO
2
+


and
rel
ated
compounds, were not found.

The space group of Amaa is used for the Rietveld analysis
for
the single phase samples of
(Bi
1.7
Pb
0.3
)(Sr
2
-
x
Pr
x
)CuO
6+


with 0.050



x


0.50. The R
wp

obtained from the Rietveld refinement o
n

this samples is between 8

10%.

Fig. 2 shows a
refinement result for x =
0.4
0

sample which is typical for all the (Bi
1.7
Pb
0.3
)(Sr
2
-
x
Pr
x
)CuO
6+


samples.

The

figures include the experimental, calculated, and difference XRD profiles.
Crosses are the experime
ntal data; solid lines are the calculated profile. All the possible
Bragg reflections are indicated with short vertical tics below the calculated profile. The
difference between the experimental and
calculated results is plotted below the Bragg
reflection
tics.

Unit cell axes as a function of x are plotted in Fig.

3
.

Both the
a
-

and
c
-
ax
is
d
e
crease with increasing x
,

while the
b
-
axis
in
creases with increasing x
,

indicating a
successfully substituting of Pr into the Sr site
.

The results of
Fig.

4

has been fo
und by
an
iodometric titration that the

oxygen
stoichiometry (
y)

and formal valence of Cu

(p)

in the (Bi
1.7
Pb
0.3
)(Sr
2
-
x
Pr
x
)CuO
6+

.

The
oxygen content was estimated by the average Cu valence under the assumption that Bi
3+
, Sr
2+
,
Pb
2+
, Pr
3+
and O
2
-

remained f
ixed valence. In the present stage, the valence of Cu indicates the
formal valence.

y

in
creases with

increasing x

from
6
.
205
(2)

for x = 0
.050

to 6.
36
9(1)

for x =
3


0.
50
.

The average hole co
ncentration (
p
) found in the

c
ompounds is in the range of 0.2
37(2)


p


0.3
6
0(3)
,

p

decrease
s

with increasing Pr content
.

Introducting Pr into the Sr site
, it

is a
hole filling effect and carrier

concentrati
on is varied
with

the Pr content
.




T
c

and hole concentration of the compound (p) in

the
BPSPCO

series relations
hip is
shown in
Fig.
5
.

T
he optimal T
c

of 20

K
with

p = 0.2
78
(2
)

is found in the sample with x =
0.40
.



A
cknow
ledgment

This work is financially supported by the National

Science Co
uncil of

Taiwan.


References

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B. Goodenough, J. Zhou, Phys. Rev. B

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ao
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an
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ang
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C. Mijhoff, G. Tendeloo,

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ain, M. Ji, J. Zhu, J. Zuo,

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4












Fig. 1.
XRD patterns

o
f

(Bi
1.7
Pb
0.3
)(Sr
2
-
x
Pr
x
)CuO
y

samples

with (0.050


x


0.50)
.


10
20
30
40
Intensity (a.u.)
2

(
o
)



Fig.

2. Rietveld refinement result of a

(Bi
1.7
Pb
0.3
)(Sr
2
-
x
Pr
x
)CuO
6+


with x = 0.
4
0. The

figures
include the experimental, calculated, and difference XRD profiles. Crosses are the
experimental data; solid lines are the calculated profile. All the possible Bragg reflections are
indicated with sho
rt vertical tics below the calculated profile. The difference between the
experimental and calculated results is plotted below the Bragg reflection tics.











Fig.

3
.
Unit
-
cell axes dependence with x

for

the

(Bi
1.7
Pb
0.3
)(Sr
2
-
x
Pr
x
)CuO
y

samples
.

0.00
0.15
0.30
0.45
5.320
5.355
5.390
24.43
24.50
24.57


Axis (A)
x
a
b
c
10
20
30
40
50
x=0.50
x=0.45
x=0.425
x=0.40
x=0.375
x=0.35
x=0.30
x=0.25
x=0.20
x=0.15
x=0.10


Intensity (a.u.)
x=0.05
2

(
o
)
5


0.00
0.15
0.30
0.45
6.20
6.25
6.30
6.35
0.20
0.25
0.30
0.35
0.40
p


y
x


F
ig.
4
.
Oxygen stoichiometry

(
y
)

and
hole concentration

(
p
)

dependence with x

for


(Bi
1.
9
Pb
0.
1
)(Sr
2
-
x
Pr
x
)CuO
y

samples
.

(
The lines drawn through

the dat
a

are
guides

to the eye
)
.


2.196
2.208
2.220
2.232
0
7
14
21


Tc
Relative intensity (Cu)

F
ig.

5
.

T
c

dependence with

hole concentration (p
) for
(Bi
1.
9
Pb
0.
1
)(Sr
2
-
x
Pr
x
)CuO
y

samples
.


(
The lines drawn through

the dat
a

are
guides

to the eye
)
.